This invention relates to a process for separating relatively magnetic mineral particles P.sub.m having magnetic susceptibility .chi..sub.m, with .chi..sub.m &gt;0, from relatively non-magnetic particles P.sub.n having magnetic susceptibilities .chi..sub.n, with .chi..sub.m &gt;.chi..sub.n, all the particles being suspended in a liquid stream.
Such a process is well known in the field of high gradient magnetic separation (HGMS) techniques, see for example the textbook by J. Svoboda, "Magnetic Methods for the Treatment of Minerals", Elsevier, 1987. Various types of separator devices are described therein. In detail they can be divided into two main classes based on their mode of operation, i.e. semi-continuous and continuous separators.
In the first type the separating container is called a canister and is typically cylindrical in shape. This canister which contains the matrix elements is usually placed in a solenoid which generates the magnetic field. The matrix elements distort this magnetic field to produce a gradient and hence a net magnetic force on the mineral particles (proportional to the product of magnetic field strength and field gradient). Pulp with the mineral mixture to be separated is fed for a given time to this canister. The magnetic mineral particles are deposited on the matrix elements. At the end of this period the feed is shut off and the canister is rinsed to displace residual lesser magnetic material from the canister. The field is then switched off and the magnetics are flushed off the matrix elements and a new cycle of feeding-rinsing-flushing can start. It is important to note that in this type of separator the canister is usually flooded, i.e. there is on no occasion an air-pulp interface. A moving interface could be problematic by unwantingly stripping off the magnetics from the matrix elements. Semi-continuous canister-type separators are widely used for purification of kaolinire for the paper industry (removal of iron oxide contaminants). The matrix in this case is stainless steel wool.
In the second type of separating device the separating containers with matrix elements are typically compartments in a horizontal carousel which rotate through the magnetic field. There are separators which do allow an interface between the pulp and the matrix elements an example being the widely used Jones separator which has grooved plates as the matrix elements and uses a conventional yoked electromagnet to generate the field. There is also a separator made by Sala in which the compartments are flooded requiring a sealing arrangement between the carousel and the stationary part which includes a solenoid magnet. The Sala separator uses either expanded metal sheets (usually in mineral processing applications other than clay purification) or stainless steel wool as the matrix.
The present invention relates primarily to the flooded separators.
There are various possible orientations of matrix elements (assumed to be elongated bodies), feed flow direction and magnetic field direction. Primarily the so-called longitudinal orientation is concerned in which the field and flow direction are mutually parallel and both perpendicular to the matrix element. In current practice in which this orientation is used conditions are such that the magnetic deposit is formed on the upstream side of the matrix element. The problem with this is that relatively non-magnetic or lesser magnetic unwanted mineral particles are mechanically entrained from the feed flow which is impinging upon the deposit of previously captured material and causes contamination of the magnetics.
However, under certain hydrodynamic conditions in the longitudinal orientation a deposit can be formed on the downstream side of the matrix elements as indicated in I.E.E.E. Transactions on Magnetics, Vol. MAG-15, Nov. 6, 1979, page 1538, "HGMS at moderate Reynolds numbers", by J. H. P. Watson, and standing vortices then form in this region as is well-known from classical hydrodynamics of flow around a cylindrical body. This phenomenon has been observed in experiments on pure phases.
If mineral separation in industrial application is concerned, such separation is usually characterized in terms of recovery of ore minerals and more commonly of contained valuable elements and grade of such minerals or elements. The recovery of a particular element is the quantity of such element reporting to the desired separation product or concentrate, expressed as a percentage of that contained in the feed. The product grade is the content of a particular mineral or element in that product usually expressed as a percentage of the total mass of the mineral or element contained in that product. In the following expressis verbis grade percentages calculated and explained are defined as mineral weight percentages.
Recovery and grade both determine the effectiveness of a separation. Their separate consideration is usually meaningless. The selectivity of a process can be expressed as the product grade of a certain element obtained at a particular recovery. The statement that one separation method is more selective than another, i.e. in the former higher grades are obtained at a specified recovery may only be valid for a particular range of recoveries. The relationship between grade and recovery for a given separation process can be evaluated experimentally and is usually such that higher recoveries correspond to lower product grades and vice versa.